This invention relates to a highly splittable polyolefin conjugate fiber and more particularly to the fiber which is suitable for the fields of industrial materials such as battery separator, wiper and filter and the like, and hygienic materials such as diaper, napkin and the like; and to a fiber structure using the same and to a process producing the fiber structure.
As is known, an islands-sea type or splittable type conjugate fiber is conventionally used to produce an extremely fine fiber. The production method using the islands-sea type conjugate fiber comprises spinning combined resins as plural components and removing one of the obtained conjugate fiber components by dissolution to give an extremely fine fiber. Although a very fine fiber can be obtained, this method is uneconomical in that one of the components needs to be removed by dissolution. On the other hand, the production method using the splittable fiber comprises spinning combined resins as plural components and splitting the conjugate fiber obtained into a large number of extremely fine fibers by utilizing a physical stress or a difference in shrinkage between the fiber-constituting resin components when immersed in a chemical.
A typical splittable conjugate fiber, for example, a combination of a polyester resin with a polyolefin resin, a polyester resin with a polyamide resin or a polyamide resin with a polyolefin resin, has poor compatibility with each other due to polymers of different kinds in the fiber, and though splitting proceeds easily, the resulting extremely fine fiber and fiber structure composed thereof have poor chemical resistance due to functional groups in the polymers. This restrains an application of these conjugate fibers to the field of industrial materials which require chemical resistance.
On the other hand, a combination of polyolefin resins each having a strong resistance to chemicals shows better compatibility than that of the different polymers as mentioned above, and therefore strong physical impact is needed to split the splittable fiber formed. This causes so-called unevenness in the resulting nonwoven such as split parts and non-split parts, or thick parts and thin parts due to a partial movement of fibers caused by the physical impact. Thus, the nonwoven shows a poor uniformity. Further, the processing speed in hydroentangling needs to be greatly reduced so as to diminish undivided parts. For these reasons, this type of combination is unsatisfactory.
JP-A-4-289222 discloses an improvement that a splittable conjugate fiber comprising polymers of the same kind is easily split by addition of organosiloxanes or modifiers thereof. Though the improvement is somewhat successful in respect of splittability, there still remain many problems such as reduction in strength of the nonwoven made with such fibers and poor processability during the secondary processing period, and the like.
An object of the present invention is to provide a polyolefin splittable conjugate fiber that is easy to split and a fiber structure with a fine surface and good uniformity using the same.
The present inventors have found, as a result of investigations, that a splittable conjugate fiber comprising at least two components of polyolefin resins, wherein at least one component containing 1-30% by weight of ethylene-vinyl alcohol copolymer whose saponification degree is 95% or more, is easy to split and that a fiber structure obtained by using the conjugate fiber has a fine surface and good uniformity. This finding has led to the present invention.
The present invention is composed of the following:
The first invention is concerned with a polyolefin splittable conjugate fiber comprising at least two components of polyolefin resins (including polystyrene resin, this also applicable hereinafter) and having a cross sectional structure with each of the components arranged alternately, wherein at least one of the component resins contains 1-30% by weight of ethylene-vinyl alcohol copolymer whose saponification degree is 95% or more.
In a preferred embodiment of the invention, each component of the polyolefin resins comprises a polypropylene resin phase and a polyethylene resin phase.
In a further preferred embodiment of the invention, at least one component of the polyolefin resins comprises a stereoregular polystyrene resin.
In another preferred embodiment of the invention, the polyolefin splittable conjugate fiber is a hollow fiber.
In another preferred embodiment of the invention, a foaming agent is added to at least one component of the polyolefin resins.
In another preferred embodiment of the invention, the splittable conjugate fiber has a bent, bulged or flat cross section.
The second invention is concerned with a fiber structure using the polyolefin splittable conjugate fiber obtained in the first invention.
The third invention is concerned with a process for producing the said fiber structure, which comprises the steps of mixing the polyolefin splittable conjugate fiber of the first invention with a fiber comprising a resin having a lower melting point, subjecting the fiber having a lower melting point to thermal bonding and subsequently splitting the said splittable conjugate fiber.